This invention relates to video special effects, and more particularly to recursive video effects that provide a "trail" or "persistence" to video images.
Recursive effects are commonly used in the video effects industry to leave a decaying after-image of an object as it moves within a field of video. Such an effect simulates a high persistence monitor, where the "persistence" is not provided by a high persistence phosphor, but rather by a digital video effect.
FIG. 1 shows a prior art circuit suitable for producing a decaying after-image or trail. Multiplier 10 multiplies incoming digital video data by one minus a decay factor, i.e., the complement of the decay factor, 1-K.sub.d. Multiplier 16 multiplies digital key and video data, separated into luminance and chrominance components, from frame delay 14 by the decay factor, K.sub.d. Summing circuit 12 adds the input video image data that has been multiplied by 1-K.sub.d, to the delayed video image that has been multiplied by K.sub.d. The resulting video output, which is normalized to one, is then fed back into the frame delay 14, as well as being used downstream from this effect. Thus, as an object in the video image moves, its decayed afterimage is recursively mixed back in with its present image, producing the desired effect, since older versions of the object's image are increasingly attenuated by the decay factor as they pass through the recursive loop.
While the form of combining means 11 that consists of multiplier 10, summing circuit 12, and multiplier 16 shown in FIG. 1 (and FIG. 2 below) is shown as a conceptually simple way to close the recursive loop, other methods are in fact frequently employed. A more sophisticated alternative is described in U.S. Pat. No. 4,851,912 to Jackson et al for "Apparatus for Combining Video Signals", hereby incorporated by reference. Using that approach, the video combination that closes the recursive loop is accomplished using a priority-key signal.
While there are presently several incidental variations on this basic approach, such as architectures that allow "strobe" effects or structures that use a field store instead of a frame store, these variations are unimportant to the practice of the invention to be described below. For reference and background, however, the reader's attention is directed to U.S. Pat. No. 4,752,826 to Barnett, for "Intra-Field Recursive Interpolator", hereby incorporated by reference.
None of the presently known variations on recursive video effects alter the color of the decaying afterimage.
In the prior art, chrominance phase rotation has been used to perform color correction and alteration. For example, U.S. Pat. No. 4,554,576 to Kao for "Auto Flesh Circuitry as for a Digital TV Receiver" describes circuitry for hue control using hue vector rotation. Similarly, U.S. Pat. No. 4,558,351 to Fling et al for a "Hue Correction Circuit for a Digital TV Receiver" also describes circuitry for hue control using hue vector rotation.